1. Field of the Invention
The present invention relates to an ink for ink-jet recording, an ink-jet recording process making use of the ink, and equipment therefor.
2. Related Background Art
As inks for ink-jet recording, those prepared by dissolving or dispersing a dye or pigment of various types in a liquid medium comprised of water or water and an organic solvent are hitherto known and are used.
To such conventional inks, it is common to add urea, urea derivatives, thiourea or thiourea derivatives as humectants (Japanese Patent Applications Laid-open No. 57-74372, No. 56-4488, No. 3-234775, No. 4-85375 and No. 4-332777). Such urea, urea derivatives, thiourea and thiourea derivatives improve solubility of dyes in various solvents and hence are used as dissolving aids (Japanese Patent Application Laid-open No. 2-173168). They are also effective for increasing ejection quantity and improving ejection performance (Japanese Patent Applications Laid-open No. 1-203483, No. 62-1765, No. 62-74973 and No. 56-88473, and Japanese Patent Publications No. 57-31759, No. 60-42834 and No. 1-6236), and acting as pH maintainers to improve storage stability (Japanese Patent Applications Laid-open No. 1-203483, No. 62-1765, No. 62-74973 and No. 56-88473, and Japanese Patent Publications No. 57-31759 and No. 1-6236.
Many of dyes, however, cause agglomeration due to ammonium ions generated as a result of decomposition of urea, so that a precipitate due to the agglomeration of dyes may be produced when inks to which urea has been added are stored for a long period of time, bringing about the problem of clogging that occurs at ink ejection outlets or ink feed pipes of ink-jet recording apparatus. In the case of inks containing a pigment, the ammonium ions generated as a result of decomposition of the urea and a dispersing agent for the pigment chemically react to cause agglomeration of the pigment, so that ink ejection outlets or ink feed pipes of ink-jet recording apparatus may clog. In addition, without regard to the types of colorants, ammonia generated by decomposition attacks metallic members or the odor of ammonia gives users an unpleasant feeling. Recently, mainly in the case of slightly water-soluble dyes, it has been also found that the addition of urea promotes a phenomenon in which printing carried out on a strongly acidic paper of about pH 4 (pH measured according to JIS-P8133) results in an increase in gloss of print characters and black has a look of gold because of the reflection of light (i.e, the bronze phenomenon or bronzing).
On account of such problems, studies have been made on ink formulation concerned with the addition of urea, where studies are made on, e.g., the improvement of moisture retention by greatly increasing the amount of a solvent (a polar solvent with a high boiling point) added, the addition of an alcohol amine as a pH maintainer, the addition of a p-toluenesulfonamide ethylene oxide addition product as a dye dissolving aid, and the prevention of agglomeration by increasing the proportion of a dispersing agent to a pigment when an ink containing a pigment is used.
However, the use of a polar solvent in a large amount, the addition of an alcohol amine or a p-toluenesulfonamide ethylene oxide addition product and the increasing of the proportion of a dispersing agent to a pigment have involved the following problems.
The addition of a polar solvent in a large amount results in a small contact angle between ink and a recording medium to cause feathering or results in a decrease in OD (optical density) to make print quality extremely poor. It also results in an increase in the viscosity of ink to make ejection performance poor.
The addition of a p-toluenesulfonamide ethylene oxide addition product is effective against the bronze phenomenon. However, like the addition of a polar solvent in a large amount, it results in a small contact angle between ink and a recording medium to cause feathering or results in a decrease in OD (optical density) to make print quality extremely poor. It also causes bubbles in ink to make recording performance poor.
The addition of an alcoholamine typified by triethanolamine, diethanolamine or monoethanolamine eliminates the problem of agglomeration of dyes or pigments during the storage of ink and the problem of the bronze phenomenon. However, because of their strong basicity, the pH of ink increases to as much as 11 to 12 even when added in an amount of about 5% by weight, so that components of various members coming into liquid-contact with ink may dissolve out of the members to cause changes in ink properties, resulting in ejection failure or ink leakage. This also results in a lowering of ink placement precision to make print quality poor. Many black dyes give a very poor tone which leads to a lowering of image quality in a multi-color print. Besides, amines are disadvantageous in that their peculiar odor gives users an unpleasant feeling.
When an ink containing a pigment is used, the viscosity of ink increases with an increase in the proportion of a dispersing agent to a pigment, and hence the sedimentation velocity decreases according to the Stokes' equation of sedimentation velocity and also the agglomeration becomes slow. Thus the problem of precipitates due to the agglomeration of pigments has been overcome. Since, however, the viscosity of ink increases, the ejection performance of ink becomes poor. In particular, in the case of ink-jet recording in which recording is carried out by ejecting ink droplets by the action of heat energy, there has been another problem in that an increase in the concentration of organic matter in the ink results in an adhesion of thermally decomposed organic matter to a heating member to prohibiting liquid droplets from bubbling.
In the meantime, though not concerned with inks for ink-jet recording, Japanese Patent Publication No. 61-55552 discloses a recording ink for writing implements which contains a humectant and a specific urea compound serving as a dissolving aid. However, compared with .inks for writing implements such as fountain pens and felt pens commonly available, inks for ink-jet recording have more strict requirements on many performances such that uniform ink droplets must be stably ejected from minute ejection orifices over a long period or short period of time. Thus, when the above specific urea compound is added in an ink with a high pH and the resulting ink is applied to ink-jet recording, no good ejection can be achieved, accompanied with ejection failure and so forth. In particular, in the ink-jet recording process making use of heat energy, this phenomenon remarkably occurs, having a great influence on printing as exemplified by image distortion due to a decrease in ejection velocity or a lowering of print density due to a decrease in ejection quantity.